A signal-processing circuit that can operate with a single supply voltage typically has a reference voltage input to which a reference voltage should be applied. In many applications, the reference voltage input should be coupled to signal ground via a relatively large capacitance, which constitutes a short circuit within a frequency range of interest. This relatively large capacitance will be referred to as reference voltage capacitance hereinafter. A reference voltage circuit imposes the reference voltage onto the reference voltage capacitance. The reference voltage is typically in between the supply voltage and signal ground.
The signal-processing circuit exhibits a transitional behavior when the single supply voltage is switched on. The same applies when the single supply voltage is switched off. This transitional behavior is connected with a charging or a discharging of the reference voltage capacitance, whichever applies. The transitional behavior may cause the signal-processing circuit to produce an undesired output signal. For example, in audio applications, the transitional behavior may cause an audible plop or click sound when switching on or switching off the single supply voltage. Such an undesired transitional behavior can be prevented by charging or discharging the reference voltage capacitance, whichever applies, in accordance with an S-curve characteristic.
US patent application published under number 2005/0084120 discloses a signal ground voltage generation circuit within an audio power amplifier. The signal ground voltage generation circuit includes a voltage follower amplifier, which is coupled between a reference voltage source and a signal ground terminal. The voltage follower amplifier outputs a current, which charges a capacitor that is coupled to the signal ground terminal. A current control circuit controls this output current so that the output current is equivalent to a maximum current.
The current control circuit comprises two differential MOSFET pairs, each of which has a gate that is coupled to the signal ground terminal. One differential MOSFET pair receives a first midpoint voltage. The other differential MOSFET pair receives a second midpoint voltage. The maximum output current is the sum of a difference between drain currents of the one differential MOSFET pair and a difference between drain currents of the other differential MOSFET pair. That is, the maximum current results from applying addition operations and subtraction operations to a plurality of currents, which vary as a function of a voltage on the signal ground terminal.